Differential-speed steering vehicles have been provided heretofore and are of a primary interest in the tracked-vehicle, tractor, military vehicle and construction vehicle class. In general they comprise a drive wheel on each side of the vehicle (left and right sides) and respective motors for operating these wheels.
The particular class of vehicle with which the instant invention is concerned are those which are driven by a hydraulic drive, e.g. a hydrostatic transmission associated with each wheel. Such drive arrangement may comprise a shiftable element, e.g. a tiltable control plate or cylinder drum or the pumpor motor or both of a hydrostatic transmission. Of course, a common pump may serve a pair of hydrostatic motors each connected with one of the drive wheels. In this case just the hydrostatic motor can have a tiltable member for controlling the driven wheel. In any event each of the driving wheels of the vehicle is associated with a hydrostatic transmission having a displaceable control element which varies the direction of rotation of the wheel and the speed thereof. The vehicle is generally provided with a prime mover such as an internal combustion engine, which can drive the pump or pumps. This control element may be operated by a servomotor (servofollower).
It is known in connection with such vehicles to provide a control lever which serves to select the direction of movement of the vehicle. For example, when the control lever or member is tilted forwardly the vehicle is expected to move forwardly with the lateral wheel being driven at the same speed. When the lever is drawn rearwardly, the drive wheels are reversed and the vehicle moves backward. When the lever is tilted to one side or the other, the driven wheels on opposite sides of the vehicle are rotated at different speeds and the vehicle is caused to turn in one direction or another. Since the wheels of the vehicle are operated at different speeds for turning, the steering system is considered a differential speed steering.
In the prior-art systems, the steering control comprises four control-pressure transmitters mounted on the vehicle in pairs equidistant from the control lever on opposite sides of the longitudinal median plane through the vehicle. Thus two transmitters lay in each of two planes equispeced from but parallel to the longitudinal median planes of the vehicle. All four parallel pressure transmitters are equispaced from the actuating lever so that each of the transmitters is actuated to the same extent for a given angular displacement of the lever in the appropriate direction.
When it is desired to drive the vehicle forwardly, the lever is swung in a plane parallel to the vehicle longitudinal median plane or in this plane itself in a forward direction.
The lever thus operates both of the forwardly disposed transmitters to the same extent. Because the two actuated control pressure transmitters have the same spring characteristics and valve characteristics, the servomotors connected to the hydrostatic drives for the wheels are displaced identically and in the same direction. The vehicle moves recitilinearly forward.
A corresponding response results when the lever is swung in the longitudinal median plane of the vehicle rearwardly from its neutral position. In this case the two rearwardly disposed transmitters are actuated identically and shift the servomotors of the hydrostatic drives in the opposite directions to the same degree so that a rectilinear reverse movement is brought about.
When the vhicle is to travel along a curve, the actuating lever is not displaced in the longitudinal median plane of the vehicle but rather is tilted at an angle thereto so that the lever is swung in a vertical plane which is inclined to the longitudinal median plane. The parallel pressure transmitters on one side thus become more effective than those on the other side. Of course when the plane in which the lever is swung is perpendicular to the longitudinal median plane through the pivot point of the lever, the vehicle rotates in place.
The conventional system thus provides a multiplicity of control conditions under the control of a single lever. For example straight forward, straight back, right hand curve forward, right hand curve back, left hand curve forward, left hand curve back, rotation in place and each with a substantially continuous range of speeds.
However, the system has the disadvantage that in the transition from curvilinear travel forwardly to curvilinear travel rearwardly the lever must be swung from one side of the controller to the other. Similarly when the sense of curvature is to be changed, the lever must be swung from one side to the other. As a consequence, successive operating conditions arise which require spontaneous and immediate reversal of the position of the lever from one extreme to the other and such movements are unnatural. In addition, there is a hiatus in the movement of the vehicle or the operation of servomechanisms of the hydrostatic transmission which can lead to failure. Furthermore in such transitions it is not possible to immediately position lever accurately in a new position which provides the desired direction and speed.
It has been found, especially in chain-driven vehicles (tracked vehicles), that even when the control lever is held still at transitions between left and right curves, the vehicle oscillates or swishtails (see German Gebrauchsmuster No. 77 16 968).